package lib.排列组合;

/*
 * Copyright 2012 wuyou (raistlic@gmail.com)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import java.math.BigInteger;

/**
 * This class is intended to provide convenient facilities for
 * frequently used maths calculations that {@code java.lang.Math}
 * does not cover.
 *
 * @date   08/08/2012
 */
public final class Calculator {

    private Calculator() {}

    private static final BigInteger[] FACT_RESULT_POOL = new BigInteger[1024];
    public static BigInteger factorial(int number) {

        if( number < 0 )
            throw new IllegalArgumentException();

        BigInteger result = null;

        if( number < FACT_RESULT_POOL.length )
            result = FACT_RESULT_POOL[number];

        if( result == null ) {

            result = BigInteger.ONE;
            for(int i = 2; i <= number; i++)
                result = result.multiply(BigInteger.valueOf(i));
            if( number < FACT_RESULT_POOL.length )
                FACT_RESULT_POOL[number] = result;
        }
        return result;
    }
}
